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Revision as of 23:29, 17 February 2013


Time: <<DateTime(2011-01-14T11:40:23-0500)>>

Drafter: JohnPurrier

Drafters Email: <<MailTo(john AT openstack DOT org)>>

Status: Proposed to the POC

Image Formats and workload portability in OpenStack.

A proposal to set a common understanding of supported disk image, cloud exchange/interoperability, and meta-data container (“virtual appliance”) formats in OpenStack and how the project can approach the issues surrounding various hypervisors, cross-cloud interoperability, and set some necessary direction for development work items.

This proposal has been published, commented on, and updated via the OpenStack mailing list.

The key point of the discussion: Virtual Disk Image Formats matter. As the longer term vision for OpenStack Compute and the cloud computing industry is formulated it is becoming clear that over the next few years the issues of portable workloads, federated cloud deployments, and seamless operations across multi-hypervisor systems (whether they are inside a single DC or federated across two or more deployments) will be a key element of success. OpenStack Compute will form the technological basis for achieving this.

However, stating that Disk Image Formats matter does not imply that there will be a “winner” in the marketplace or that a singular format should be pushed via the OpenStack project. In order to achieve the goals stated above surrounding workload portability the key element will be not the actual virtual disk formats, but rather a VM interchange standard and the ability to easily re-purpose the VM image to the target environment (whether it be a public cloud or a private instance).

Today we are seeing most, if not all, of the popular virtualization systems branching to support alternate VM disk image formats beyond what they consider “native”, either through conversion tools or the ability to natively mount and run the alternative formats. This demonstrates that the market understands the importance of being able to be somewhat disk image format agnostic and that there is no value in trying to lock-in customers through proprietary formats.

The Virtual Disk Format playing field today looks like:

  • VHD – Citrix XenServer and Microsoft Hyper-V
  • VMDK – VMware ESX
  • VDI – Oracle VirtualBox ·
  • QCOW2 – KVM and QEMU

All the hypervisors also support RAW Disk Images. Additionally, deployments can combine a disk partitioning and file-system (usually via LVM) to get more direct control of RAW Images and to provide some additional features, such as Snapshot with Coalesce (as Rackspace does on the current Linux Cloud Servers product).

These disk image formats are not too complicated, and there are free and cheap tools that allow conversions between the various formats. As stated above, there is a movement within all of these projects to reduce the friction in moving VM images into an environment that were created by someone else.


OpenStack has, at its core, a mission to provide widespread ubiquity of Virtual/Cloud Computing and also has taken a hypervisor-agnostic approach. In order to foster this and to move the project to a world where workloads can be easily moved between installations that are running different underlying virtualization systems the following should occur:

  1. A standardized VM Disk Image exchange format should be described. The DMTF already has a specification that is useful here, OVF. OVF is a specification that provides for an exchange container that will include standardized mechanisms to describe meta-data and also to encapsulate one or more VM images or .ISO files. OVF does not dictate any particular VM Image format, and hence is ideal for our purposes.
A variant of OVF, called OVA provides a single-file, compressed, version of the OVF format. Another term for this set of envelope, metadata, and disk image content is a “virtual appliance”.
There is an existing specification that is necessary to provide in order  to maintain EC2 compatibility; this is the Amazon AMI virtual appliance  format. This should be taken into account while drafting the supported OpenStack virtual appliance requirements and specification.
An OpenStack VM interchange/virtual appliance specification should be drafted and reviewed by the OpenStack community. 
  1. OpenStack should *not* specify a "preferred" or "default" virtual disk format. Just as a core tenet is hypervisor agnosticism another key position will be VM Disk Image agnosticism.
The message is that each deployment should choose the best hypervisor and image format for their particular goals. This allows companies like Rackspace or others to choose the systems and formats that best support the features they want to take to the market. This also sidesteps the potentially divisive topic of support for VHD and the Microsoft Open Promise licensing. Since support will be optional for all formats if someone is adamant about not supporting VHD in their OpenStack deployment they are free to not include it. Others, including Rackspace, may choose to support VHD for the features it provides.
||Virtual Disk Images Supported||OpenStack Exchange / Virtual Appliance Formats||
||VHD, VMDK, VDI, QCOW2, RAW ||OVF (schema defined by OpenStack), OVA, AMI ||
  1. The OpenStack Image Repository project (“Glance”) should be extended to provide native disk image and cross-cloud exchange/virtual appliance conversion capabilities. This will hide the differences in formats from the deployment and operational decisions and allow workloads to easily be imported and exported between both OpenStack installations, and also to/from native virtualization installations (such as VMware).